This invention pertains to the manufacture of solid state electronic devices and more particularly to improvements in the method of fabricating polycrystalline silicon solar cells wherein the damaged surface layer generated during hydrogen passivation is used as a plating mask for the metallization of the front surface electrodes.
Heretofore a common method of fabricating silicon solar cells has included the steps of forming a PN junction by diffusing a suitable dopant into the front side of a silicon wafer or ribbon, etching a grid electrode pattern in a protective dielectric masking layer formed on that front surface, depositing a nickel plating on all silicon exposed by the etching, solder dipping or overplating the nickel with copper and tin, removing the remainder of the dielectric masking layer from the front surface, and providing an antireflection coating on the newly exposed portions of the front surface.
While such a procedure may be applied to either single crystal or polycrystalline silicon, cost considerations make it desirable to fabricate solar cells from the latter. However, as is well known, because of the minority carrier losses at grain boundaries, dislocations, and the like, the efficiencies achieved with polycrystalline silicon solar cells are generally poorer than those of monocrystalline cells. This circumstance has been improved upon by introducing a monovalent element, such as hydrogen, into the polycrystalline material to combine with the dangling bonds associated with the structural defects, thereby minimizing the minority carrier recombination loss.
A variety of manufacturing protocols have been developed to integrate a hydrogen passivation step into the volume manufacture of solar cells. For instance, as taught in U.S. patent application Ser. Nos. 563,061, 563,292, and 563,132, all filed Dec. 18, 1983, the hydrogen passivation may be incorporated into the manufacturing process so as to form a plating mask for the control of subsequent metallization of the front surface electrodes. Thus, as described in U.S. patent application Ser. No. 563,061, a preferred embodiment of the process described in detail therein as applied to the manufacture of silicon solar cells involves, inter alia, the following steps: (1) forming a plating mask of a dielectric material on the front surface of a shallow-junction silicon ribbon so as to leave exposed those areas of the silicon to be covered by the front surface electrode, (2) depositing a thin layer of nickel (or similar material) on the exposed silicon, thereby forming an initial metal layer in the electrode pattern, (3) removing the plating mask, thereby exposing the silicon between the initial metal layer of the front electrode, (4) hydrogen ion-beam passivating the junction side of the cell in such a way as to form, inter alia, a fresh plating mask on the silicon between the electrodes, (5) sintering the nickel to form in part a nickel silicide, (6) plating additional metal(s) onto the metal-covered portions of the cell, and (7) antireflection coating the exposed surface of the silicon. Thereafter, the silicon may be further processed, e.g. to prepare it for connection to electrical circuits. In an alternative process, the heating of the sample during passivation supplies at least part of the energy for the nickel sintering step. Related proceedures are taught in U.S. patent application Ser. Nos. 563,292 and 563,192.
The exact nature of the altered surface layer formed as a result of ion-beam passivation, and forming the plating mask, is not known. One possibility is that the layer is a damaged zone wherein the crystal structure has been somewhat disrupted, the silicon in part forming SiH or SiH.sub.2 with hydrogen from the ion beam, yet wherein the material is possibly amorphous. It has also been suggested that carbon vapor or a hydrocarbon inevitably present in the vacuum system might form a dielectric layer on the silicon surface. Whatever its nature, it was found that a plating mask could be formed concurrently with the hydrogen passivation of EFG-grown silicon by a hydrogen ion beam of a Kaufman-type (broad beam) ion source.
As noted in all of the U.S. patent applications referred to above, a small amount of carbon or of one or more hydrocarbons appears to be necessary for the consistent formation of an altered surface layer of the desired quality. The quantity of carbon or hydrocarbon apparently necessary was also apparently readily available from the inevitable contaminants in the ion-beam vacuum system. Thus, the grid of the Kaufman-type ion source is typically of graphite. Also, as first used, the vacuum system was equipped with a graphite mounting stage about 5 inches (c. 13 cm) in diameter on which the substrates, typically 2 by 4 inches (5 by 10 cm) on a side, were centrally located. It was noted that when a silicon mounting stage was substituted for the graphite stage, the altered surface layer formed by the ion bombardment did not perform as a plating mask as consistently as when the graphite stage was employed. This observation gave rise to the hypothesis that carbon vapor or hydrocarbon produced, for instance, by the impact of the ion beam on the graphite stage might enter into the formation of the altered layer.
While the methods disclosed in the referenced U.S. patent applications resulted in simplified manufacturing processes, there was nevertheless an undesirable loss of partially finished cells with poor ion beam-formed plating masks. Even with the graphite stage, results were not completely consistent. The yield of substrates with suitable plating masks, while high, was not as high as was desired for economical production of cells.